1. Field of the Invention
The present invention pertains to rigid polyurethane and polyisocyanurate foams. More particularly, the present invention pertains to such foams prepared employing as the blowing agent, a ternary blowing agent system containing HCFC-22, water, and either or both of methylene chloride or 1,1,1-trichloroethane.
2. Background of the Invention
Due to environmental concerns, the use of chlorofluorocarbons (CFC's) as blowing agents for plastic foams has come under increasing scrutiny, and such blowing agents are scheduled to be phased out early in the next decade. In the United States, such blowing agents are also subject to increasing taxation to deter their use.
Water has become an acceptable substitute for CFCs in a variety of polyurethane foams, particularly for polyurethane flexible foams. However, in rigid foams, where low thermal conductivity is an important factor, use of water alone as a blowing agent produces foams containing carbon dioxide in the cells, resulting in foams with undesirably high k-factors. The use of water in conjunction with blowing agents having low ozone depletion potential such as monochlorodifluoromethane (refrigerant 22, R-22, HCFC-22) has improved k-factors measurably, however the dimensional stability of such foams often suffers, particularly at low densities, and the aged k-factors also could stand improvement. Finally, since HCFC-22 is a gas, the amount of HCFC-22 useable in a formulation is often limited due to the lack of pressurized tanks and other equipment to relatively low amounts, c.a. 1-5 weight percent. At this low level, appreciable amounts of water must be utilized to keep foam density down. Use of pressurized equipment can minimize this drawback, but the foams produced with high levels of HCFC-22 are undesirably flammable.